1. Field of the Invention
The present invention relates to a process for preparing acrylate esters, methacrylate esters, polyester acrylates or polyester methacrylates by reacting acrylic or methacrylic acid with a monohydroxy containing compound or a polyhydroxy containing compound in a reaction vessel in the presence of a catalyst and polymerization inhibitor.
2. Description of Related Art
Conventionally, methacrylate esters and acrylate esters have been prepared via a direct esterification reaction by reacting methacrylic acid or acrylic acid, usually in excess, with an alcohol or a polyol in the presence of an azeotroping solvent for water removal, typically a hydrocarbon solvent, an esterification catalyst and a polymerization inhibitor. Limited examples are also known where synthesis can also occur in the absence of a hydrocarbon solvent, for example, European Patent Application 0 202 610 by BASF Corporation and European Patent Application 0 376 090 by Henkel Corporation. In addition, (meth)acrylate esters have been prepared using transesterification reaction conditions as described in U.S. Pat. Nos. 5,498,751 and 5,554,785.
In all of the above mentioned patents and published applications, the source of heating is conventional thermal methods, such as placing a heating unit into the reaction mixture or jacketing the reaction vessel and reaction mixture with an external heating source. In conventional thermal methods, the reactants are placed in a vessel, the vessel is heated by conventional thermal methods, and the heat is transferred to the reaction mixture conductively, usually in the presence of a solvent.
Microwaves have been used as an energy source in various esterification reactions where reactants are not prone to undergo further reaction via free radical initiated propagation. For example, U.S. Pat. No. 5,239,017 to Pelesko et al. discloses that anhydride groups of a polymer can be esterified in a cross-linking reaction with a polyol using microwave heating.
A working example in U.S. Pat. No. 5,387,397 to Strauss uses microwave heating to react certain unsaturated acids with an alcohol. This patent also couples the reactor directly to refrigeration for cooling in order to prevent by-product formation or product degradation. Further, water produced by the esterification reaction in the example is not separated from the reaction mixture. The rapid heating and cooling is to prevent decomposition or polymerization of the product, and is disclosed as an advantage. The Strauss patent does not disclose that methacrylic acid or acrylic acid can be esterified using microwaves as the energy source, but only discloses that acids with internal hydrocarbon chain unsaturation, such as crotonic acid, can be esterified. It is well known in the art that internal unsaturation is generally more stable to thermal or free radical induced reactions than terminal unsaturation in unsaturated compounds, such as those terminated with (meth)acrylic functionality.
Raner et al., J. Org. Chem., 57, 6231-6234 (1992), compares the properties and parameters of the esterification reaction of 2,4,6-Trimethylbenzoic Acid with 2-propanol using as a heat source, microwaves, verses a conventional heat source. Raner et al., J. Org. Chem., 58, 950-953 (1993), continues the investigations of the preceding article. These articles do not disclose the esterification of (meth)acrylic acid.
Strauss et al., Aust. J. Chem., 48, 1665-1692 (1995), reviews microwave assisted organic synthesis. On pages 1677-78 of this article, a discussion of the further reaction of a terminally unsaturated acrylate compound using microwaves is presented. This article does not disclose that the particular (meth)acrylic acid or acrylic acid material can be prepared by using a microwave, only that it can be further reacted with additional reactants via microwave energy. Additionally, like the patent to Strauss discussed previously, this article indicates a need to rapidly cool the reaction mixture in order to limit hydrolysis, dimerization and polymerization of the resulting product containing the unsaturation.